The Biological Nature of Soil

Welcome to Book of the Week – a weekly feature offering you a glimpse between the pages of an Acres U.S.A. published title. Get the Book of the Week email newsletter delivered directly to your in box! This week’s Book of the Week feature is The Farm as Ecosystem, by Jerry Brunetti.

Soils contain an estimated 2–3 million species of bacteria and an estimated 1.5 million species of fungi, and only 2–5 percent have been described or named! We have a robot searching for life on Mars while we ignore or marginalize the most significant life-forms that allow this planet to be alive. As in animals, the vast majority of these microbes dwell in the “digestive system” of the plant, the root ball or the rhizosphere. Also as with animals, the majority of the immune system of plants is in the “gut” because high populations of beneficial, symbiotic microbes are both the plants’ and animals’ primary first line of defense to synthesize compounds that not only protect the digestive system or root ball/rhizosphere but also supply compounds that can be taken up by plants to protect its tissues above ground. Generally we refer to these plant protectants as plant secondary metabolites, compounds that are beneficial to the plant but are not directly necessary for its survival. Soils that have a well-balanced soil food web, have excellent physical and mineral properties, and depend upon biodiversity and high populations of these diverse microbial species are typically referred to as “immune soils.” Practices that can enhance the disease-suppressive properties of soils are:

liming acid soils
compost
proper irrigation and water quality (pH, TDS, bicarbonate, alkalinity, salinity, EC, etc.)
aeration and drainage
reduced or zero tillage
cover crops (preferably multiple biodiverse species)
balanced fertilizer applications
tight crop rotations
crop residues returned to surface or shallow “sheet composting”
seed inoculation with mycorrhizal and trichodermal fungal spores, etc.

According to Brady and Weil’s The Nature and Properties of Soils, the reproductive potential of bacteria is incomprehensible. A single bacteria allowed to divide every hour would yield 17 million cells in twenty-four hours. In six days this reproductive juggernaut would yield a volume of organisms greater than the volume of the Earth! Of course, bacteria being a primary food for everyone else, this situation will never be anything but a fantastic hypothesis, but it does impress upon one the amazing volume of “livestock” that make up life in the soil, and how much energy and fertility these “livestock” contribute to the productivity and well-being of natural systems, be they forest or farm.

Aerobic organisms utilize free (atmospheric) oxygen for their metabolism. Zero to six inches below the soil is where most of the microbial activity is occurring. This fact is often referred to as the fence post principle, which states that most microbial activity happens at the point where an untreated fence post will decompose, beginning at the soil surface and extending a few inches below the surface.

Atmospheric gases are typically 21 percent oxygen and 250–350 ppm or 0.025–0.035 percent carbon dioxide (CO2 ). The soil atmosphere is generally about 15 percent oxygen and 2,000–4,000 ppm or 0.2–0.4 percent carbon dioxide. The exhalation of soil life creates this difference and, of course, that CO2 is a primary raw material that plants recycle through their stomata to incorporate into the chloroplast. There it can combine with water (H2 O) to create sugars (CHO), which are in turn the building blocks of numerous other carbon complexes, such as starches, cellulose, hemicelluloses, lignin, waxes, oils, resins, pectins, fructans, glucans, and numerous plant secondary metabolites like terpenes, alkaloids, and phenols.

The photo shows me drawing the trapped soil atmosphere out of a four-inch diameter cylinder tapped into the soil surface and capped. This is allowed to sit overnight before I draw out the air and gases that have accumulated in that containment. Halfway between the cap and the syringe is a CO2 monitoring indicator capsule that captures the CO2 in the trapped atmosphere and registers the amount via a color code. Needless to say, the CO2 amounts generated in soils that are either pastured or cover cropped versus soils that have been subject to continuous tillage without any cover are significantly different. Ninety percent of the carbon dioxide produced by life on this planet primarily originates from bacteria and fungi. According to James Nardi, a research scientist at the University of Illinois at Urbana-Champaign, a single acre of rich soil can easily surpass the metabolic activity of fifty thousand human beings.

Brady and Weil indicate in their book The Nature and Properties of Soils that the “living (liquid) carbon” is retained more in the soil than is the carbon (crop residue) that we have been mostly focused on, since on the average two-thirds of the crop residue is going to be discharged as CO2 gas. That’s why, when growing annuals, it is important not only to have their residues digested in the soil but also to get that bare ground covered quickly with living plants.

About the Author:

Jerry Brunetti,1950-2014, worked as a soil and crop consultant, primarily for livestock farms and ranches, and improved crop quality and livestock performance and health on certified organic farms. In 1979, he founded Agri-Dynamics Inc., and confounded Earthworks in 1990. He spoke widely on the topics of human, animal and farm health.